| Summary: | 碩士 === 國立勤益科技大學 === 化工與材料工程系 === 106 === The sensing performances of a planar amperometric formaldehyde gas sensor based on the porous Pt electrode prepared with the hydrogen bubble dynamic template (HBDT) technique, and cast with Nafion film as the solid polymer electrolyte (SPE) are investigated in this thesis. The effect of temperature, concentrations of H2SO4 and H2PtCl6, charge passed (Qpt) and current density for preparing Pt sensing electrode on the characteristics and the sensing properties of the sensing electrode is investigated by using the conventional method. The conditions for the preparation of Pt sensing electrode are optimized by using the mixture design technique. The surface morphologies and crystallographic information of as-prepared electrode materials are analyzed by the FESEM and XRD, respectively. The electrochemical properties are investigated by the cyclic voltammetry (CV) and chronoamperometry technique.
Based on the investigation with the conventional experimental technique, the maximum specific sensitivity of the amperometric HCHO gas sensor is obtained to be 20.37 μA ppm-1 mg-1 due to the higher specific electrochemical surface area (SECSA) (22.89 m2 g-1) of the porous Pt sensing electrode prepared in 0.0075 M H2PtCl6 and 0.05 M H2SO4 aqueous solution at 30 oC with QPt and the current density of 6 C and 0.075 A cm-2. The experimental results reveal that the specific sensitivity of the HCHO gas sensor are mainly affected by the concentration of H2PtCl6, Qpt and temperature for preparing Pt sensing electrode, which are optimized by using the mixture design technique. The relationship between the specific sensitivity of the HCHO gas sensor (Y) and the normalized [H2PtCl6] (X1), Qpt (X2) and temperature (X3) for preparing Pt sensing electrode is obtained to be
Y = 17.24 X1 + 8.35X2 + 13.30X3 + 25.63X1X3
The maximum specific sensitivity of the HCHO gas sensor was obtained to be 23.44 μA ppm-1 mg-1 for preparing the Pt sensing electrode in 0.007 M H2PtCl6 and 0.05 M H2SO4 aqueous solution with QPt and the current density of 3.0 C and 0.075 A cm-2 at 31 oC. The mass transfer resistance of the HCHO gas sensor within the gas phase can be neglected for gas flow rate greater than 450 ml min-1. When the gas flow rate is fixed at 450 ml min-1, the specific sensitivity of the amperometric HCHO gas sensor is increased from 38.05 to 43.85 A ppm-1 mg-1 by decreasing the Nafion film thickness from 2.84 0.08 to 1.18 0.10 m due to the decrease in the mass transfer resistance within the Nafion film. The diffusivity of HCHO within the Nafion film is found to be 2.33 x 10-9 cm2 s-1. The fading fraction of the specific sensitivity of Nafion®/Pt/Au/Al2O3 sensing electrode is 32.3% for 5 sensing runs due to the formation of intermediates CO(ads) and HCOOH(ads) on the Pt sensing electrode surface.
The effect of 1.33 ppm acetone, 70.71 ppm benzene, and 0.039 % CO2 on the selectivity of Nafion®/Pt/Au/Al2O3 sensing electrode is slight. The selectivity of the HCHO gas sensor is affected by the presence of 35 ppm ethanol and 20% O2 due to the oxidation of ethanol and reduction of O2 on the sensing electrode. The response time (t90) and detection limits of the Nafion®/Pt/Au/Al2O3 sensing electrode are obtained to be 240-325 s and 20 ppb, respectively, which is less than the legal levels of formaldehyde in the indoor air defined by WHO (World Health Organization) (80 ppb) for 30 min exposure limit.
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